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|- bgcolor="#FFFAFA" | Note (category: variability): || H and K emission vary. Arcturus (; α Boo, α Boötis, Alpha Boötis) of the constellation Boötes is the brightest star in the northern celestial hemisphere. With a visual magnitude of −0.05, it is the fourth brightest star in the night sky, after −1.46 magnitude Sirius, −0.86 magnitude Canopus, and −0.27 magnitude Alpha Centauri. It is a relatively close star at only 36.7 light-years from Earth, and, together with Vega and Sirius, one of the most luminous stars in the Sun's neighborhood. Arcturus is a type K0 III orange giant star, with an absolute magnitude of −0.30. It has likely exhausted its hydrogen from its core and is currently in its active hydrogen shell burning phase. It will continue to expand before entering horizontal branch stage of its life cycle. ==Observational history== As one of the brightest stars in the sky, Arcturus has been significant to observers since antiquity. In Mesopotamia, it was linked to the god Enlil, and also known as Shudun, "yoke",〔 or SHU-PA of unknown derivation in the ''Three Stars Each'' Babylonian star catalogues and later MUL.APIN around 1100 BC. Ptolemy described Arcturus as ''subrufa'' "slightly red", and it has a B-V color index of +1.23, roughly midway between Pollux (B-V +1.00) and Aldebaran (B-V +1.54).〔 Prehistoric Polynesian navigators knew Arcturus as ''Hōkūleʻa'', the "Star of Joy". Arcturus is the zenith star of the Hawaiian Islands. Using Hōkūleʻa and other stars, the Polynesians launched their double-hulled canoes from Tahiti and the Marquesas Islands. Traveling east and north they eventually crossed the equator and reached the latitude at which Arcturus would appear directly overhead in the summer night sky. Knowing they had arrived at the exact latitude of the island chain, they sailed due west on the trade winds to landfall. If Hōkūleʻa could be kept directly overhead, they landed on the southeastern shores of the Big Island of Hawaiʻi. For a return trip to Tahiti the navigators could use Sirius, the zenith star of that island. Since 1976, the Polynesian Voyaging Society's ''Hōkūle‘a'' has crossed the Pacific Ocean many times under navigators who have incorporated this wayfinding technique in their non-instrument navigation. The French mathematician and astronomer Jean-Baptiste Morin observed Arcturus in the daytime with a telescope (a first for any star other than the Sun and supernovae) in 1635, and Arcturus has been seen at or just before sunset with the naked eye.〔 Based upon an annual parallax shift of 88.83 milliarcseconds as measured by the Hipparcos satellite, Arcturus is from Earth—relatively close in astronomical terms. The margin of error is 0.54 milliarcseconds, translating to a margin of error of ±.〔 Together with Spica and Denebola (or Regulus, depending on the source), Arcturus is part of the Spring Triangle asterism, and by extension, also of the Great Diamond after factoring in Cor Caroli. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Arcturus」の詳細全文を読む スポンサード リンク
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